These are abbreviated lexicons. For example, the Perseus online lexicon (sometimes brutally slow, and also abbreviated so that reverse searches are necessary, and tends not to let me search the same term twice in one day) also gives kinswoman, cousin (iirc) and bride as definitions of adelphi. Cambridge University Classics dept. intends to add their database to Perseus in 2010. These databases cull the definitions in part from the extant texts that are posted on the database. Thus, as more extant Greek literature is added, the definitions can expand.
Perseus Table of Contents
Note, for example, that in extant Greek secular works, the use is also applied to neighbor (though I can't recall the author, sorry). When Plato uses the term "adelphn", in the Laws, he gives a further explanation -- of the same mother, for ex. (In Plato, because the particular meaning is explained by narrowdescription, Perseus still gives the definition "sister" as - due to Platos' narrowing - the particular textual use of adelphn is thus known; this does not mean that adelphn always means sister).